Most bullets make small sonic booms when flying through the air, which to our ears sound like a loud, distinct “crack!”
For the Pentagon’s special forces, that makes it hard to be sneaky
about what they’re shooting. Now the commandos want to be sneakier with
slower, quieter bullets.

In its latest round of small-business solicitations, the Pentagon’s
Special Operations Command, or SOCOM, is seeking out subsonic
ammunition. The reason, according to the solicitation, is to ”provide superior covert and stealth capabilities”
for not only the military, but police forces and the Department of
Homeland Security. In theory, and for rifles in the 5.56, 7.62 and .338
calibers, the bullets will travel at low enough velocities to avoid
breaking the sound barrier, thus creating no “crack” noise. Breaking the
sound barrier also pretty much negates the use of a sound suppressor,
or “silencer,” which the special forces would likely want to use against militants in Afghanistan and around the world.

At present, the Defense Department does not have subsonic bullets
“classified for use in the calibers provided by any DoD service.” That
doesn’t mean special operations forces never use them. Commandos have used subsonic bullets since World War II,
though these are mainly effective in smaller guns like the .22 and 9 mm
caliber pistols. Subsonic bullets and fairly large-caliber war rifles,
on the other hand, don’t mix very well.

For one, to keep a bullet from breaking the sound barrier — 1,100
feet per second at sea level — requires several trade-offs at higher
calibers. According to the solicitation, subsonic bullets “experience
significant accuracy problems due to excessive deviations in velocity.”
The gunpowder (or propellant charge) for a subsonic bullet has to be
used in smaller quantities than for a normal bullet, and the bullet
itself has to be heavier. This results in bullet that is far and away
less accurate, doesn’t go nearly as far, and “creates lower pressures
which … makes it hard to get a clean burn of the propellant causing
rapid fouling of the weapon.”